Custom properties on an action, any place for UI visibility?
Can also add a custom property panel to the Action Editor, much the same way as it is added to other parts of the UI.  Import the rna_prop_ui.PropertyPanel class and set the path from context to "object.animation_data.action" and the type to bpy.types.Action.  

The default bl_label if not defined otherwise is "Custom Properties".



import bpy

from bpy.types import Panel
from rna_prop_ui import PropertyPanel


class ACTION_PT_custom_props(PropertyPanel, Panel):
    bl_label = "Action Properties"
    bl_idname = "ACTION_PT_custom_props"
    bl_space_type = 'DOPESHEET_EDITOR'
    bl_region_type = 'UI'
    # required for PropertyPanel
    #COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_GAME'}
    _context_path = "object.animation_data.action"
    _property_type = bpy.types.Action

    @classmethod
    def poll(self, context):
        obj = getattr(context, "object", None)
        ad = getattr(obj, "animation_data", None)
        action = getattr(ad, "action", None)
        return action is not None

def register():
    bpy.utils.register_class(ACTION_PT_custom_props)

def unregister():
    bpy.utils.unregister_class(ACTION_PT_custom_props)

if __name__ == "__main__":  # only for live edit.
    register()